In general, the output power of a radio transmitter is varied by changes in the voltage of the power supply, the ambient temperature, RF load, etc. To avoid this undesirable situation, an automatic power control circuit has been used to stabilize the output power.
A conventional apparatus of this kind for controlling the output power of a transmitter is shown in FIG. 4. This apparatus has a power amplifier 1, a coupler 2, an RF amplifier 3, a detector 4, a comparator 5, and a voltage control circuit 6. These components 1-6 constitute an automatic power control loop, abbreviated APC loop. Radio signals are received at an input terminal 7. Also included in the apparatus are an antenna ANT and a reference voltage terminal 8.
When the output power of the transmitter is varied due to changes in the ambient temperature, for example, the coupler 2 delivers an output corresponding to the variation. This output signal is then amplified by the RF amplifier 3. Then, the detector 4 produces an output corresponding to the varying output power. The comparator 5 delivers an output proportional to the difference between the output from the detector 4 and the reference voltage applied at the reference voltage terminal 8. The output voltage from the comparator is supplied to the voltage control circuit 6 to control the voltage of the power supply for the power amplifier 1, for maintaining the output power of the transmitter constant.
In mobile radio communication, the output power of a mobile station is exponentially increased or reduced according to the electric field produced by receiving the waves from a base station, i.e., according to the distance from the base station, because it is effective in suppressing the reception sensitivity at the base station and removing transmission intermodulation. Further, it is preferable in that it curtails the electric power consumed by the mobile station itself. Therefore, this is the method generally adopted. Even if this method is employed, it is required that the output voltage from the detector 4 be restricted to a certain range and that the comparator 5 be operated within a given dynamic range to properly operate the APC loop. For this reason, an RF variable attenuator 9 is inserted between the coupler 2 and the RF amplifier 3. An output power setter 10 for switching the amount of attenuation introduced by the attenuator 9 between a plurality of values is connected to the attenuator 9. An input terminal 11 at which a control signal is applied is connected to the setter 10.
Another conventional apparatus is shown in FIG. 5. It is to be noted that like components are denoted by like reference numerals throughout FIGS. 4 and 5. Instead of the RF variable attenuator and other components shown in FIG. 4, this apparatus uses a first resistive potential divider consisting of two resistors R.sub.1 and R.sub.2, a first switch circuit 12, a second resistive potential divider consisting of three resistors R.sub.3, R.sub.4, R.sub.5, a second switch circuit 13, and a switching device 14. This device 14 determines the output power of the transmitter, based on the intensity of the electric field produced by receiving waves from a base station, and controls the switching action of the switching circuits 12 and 13. A terminal 15 is provided for connection with a power supply. Each of the first and second switch circuits 12 and 13 incorporates a parallel combination of a plurality of resistors (not shown). A switch is connected to each of these resistors. The output from the switching device 14 turns on some of these switches and switches from some resistors in the switch circuit 12 or 13 to other resistors. When the output power of the transmitter is maintained in steady state, the output from the switching device 14 turns on the first switch circuit 12 so that the amount of attenuation introduced by the first resistive potential divider (R.sub.1, R.sub.2) may be set to a large value. Thus, the voltage supplied from the detector 4 into the comparator 5 is limited. At the same time, the amount of attenuation introduced by the second resistive potential divider is set to a small value. When the output power of the transmitter is set to a low level, the second switch circuit 13 is turned on in such a way that the amount of attenuation introduced by the second resistive potential divider is set to a large value. As a result, the reference voltage is set to a low level. The output voltage from the demodulator 4 is applied to the comparator 5 via the resistor R.sub.1. In this way, the APC loop functions properly both during steady-state transmission and during low-level transmission.
In the conventional apparatus shown in FIG. 4, even during the transmission at the maximum power level, the output level from the detector 4 is required to be restricted to a certain range. Accordingly, the maximum amount of attenuation introduced by the RF variable attenuator 9 must be set to a large value. This makes the attenuator 9 complex and bulky. When the output power of the transmitter is at a low level, the voltage from the coupler 2 is greatly attenuated by the attenuator 9 and so the RF amplifier 3 must have a large amplification factor. Thus, the number of stages of amplification is large, making the circuitry complex. Since the attenuator 9 is designed to exhibit a large maximum amount of attenuation, it is greatly affected by other RF radiation. Therefore, the attenuator 9 must be shielded, or other countermeasure must be taken. Further, since the RF amplifier 4 has a large amplification factor, it must be shielded or otherwise processed to prevent it from being affected by other radiation and prevent abnormal oscillation. In this way, this apparatus has various problems.
In the conventional apparatus shown in FIG. 5, when the parallel combination of resistors and switches included in the first switch circuit 12 is made up of a small number of components, if the output power of the transmitter is switched to other level so that it may be varied exponentially, the detector voltage applied to the comparator 5 also varies exponentially. Therefore, it is also necessary that the reference voltage be increased or reduced exponentially. This makes it difficult to appropriately set the values of the resistors R.sub.3, R.sub.5, etc. included in the second resistive potential divider. Further, since the resistance values are susceptible to change due to temperature increase or for other cause, the stability of the output power is poor. When the first switch circuit 12 including the resistors and switches is made up of a large number of components, it is easy to set the resistance value of the second resistive potential divider. However, the configuration of the first switch circuit 12 is made complex.